1. Field of the Invention
The present invention relates to a method for remediation of organic and mixed wastes by mediated electrochemical oxidation of the organic components to carbon dioxide using a Co(III) mediator in a nitric acid solution.
2. Description of Related Art
Destruction of organic waste through conventional methods such as incineration often encounters political or regulatory hurdles, especially when the waste streams contain radionuclides or chlorinated hydrocarbons, or when harmful species such as dioxins or furans are possible by-products. For this reason, attention has focused on low temperature alternatives. Mediated Electrochemical Oxidation (MEO) is an alternative that avoids many of the problems associated with other organic or mixed waste treatment systems.
MEO oxidizes the organic components of waste to carbon dioxide and water while operating at near ambient pressure and temperature (30-70.degree. C.) in an aqueous system. All waste stream components and oxidation products (except evolved gases) are contained in the aqueous environment. The aqueous electrolyte acts as an accumulator for inorganics that were present in the original waste stream and also provides a thermal buffer for the energy released during oxidation of the organics. The generation of secondary waste is minimal as the process needs no additional reagents. The entire process can be shut down by simply turning off the power, avoiding problems such as thermal runaway.
MEO is based upon the oxidation of organics to carbon dioxide through a series of charge transfer steps involving a mediator. This mediator, typically a transition metal in its highest valency state, is generated at the anode of an electrochemical cell and is dispersed throughout the anolyte solution. Upon oxidation of the organic by the mediator, the reduced mediator species is reoxidized at the anode and the cycle repeats until all oxidizable organic material in the cell is depleted.
Several high-valency metal ions have been investigated as mediators, including Ag(II), Ce(IV), and Fe(III). Each of these systems show different rates with respect to organics oxidation, as well as slightly different optimal waste streams. The silver/nitric acid based system arguably has been the most successful of the three, yet the Ag(II) mediator suffers from the problem of being depleted when chlorinated organics are treated, since AgCl is highly insoluble. In addition, the economic and environmental costs of silver are significantly higher than for the other metals.
A cobalt based system was developed by Farmer et al. at Lawrence Livermore National Laboratory and is disclosed in U.S. Pat. No. 5,516,972. This system uses a sulfuric acid (H.sub.2 SO.sub.4) electrolyte and has no cell separator. The choice of a Co(III) mediator is advantageous when treating chloro-containing organics because cobalt chloride complexes are generally soluble, so precipitation of chloride salts is avoided. The rate of reaction of Co(III) with water is relatively slow, thus the parasitic reactions of the mediator with water are minimized. In addition, the economic and environmental costs of cobalt as a mediator are smaller than for other metals, particularly silver. Specifically, the RCRA disposal restriction requirements, both at the federal and state level, are less strict for cobalt than for silver.
Yet the Co(III)/(H.sub.2 SO.sub.4) system described in U.S. Pat. No. 5,516,972 has its own drawbacks, which were uncovered upon further study of the system. Although the system has the seemingly attractive feature of no cell separator, the disadvantage of such a system is that the organic waste stream is diluted in the larger, unseparated volume, and more reagents are required to achieve effective concentrations. Furthermore, corrosion of the electrodes was a significant technical problem for this process, as the combination of acidic media, highly oxidizing species, chlorine species, free radicals, and organic fragments drastically shortened the usable lifetime of the electrode materials tested. The sulfuric acid electrolyte solution was identified specifically as a corrosive environment that readily attacks and dissolves the anode. The corrosion of the electrodes is significant not only because it reduces the lifetime of the electrodes, which are typically made of expensive metals, but it causes the electrode material to be released into the waste stream. Waste contaminated with an electrode material such as platinum is hazardous; waste disposal or recovery of the platinum are time-consuming and costly options.
It is the object of the present invention to address the problems inherent in the Ag(II) and Co(III)/(H.sub.2 SO.sub.4) systems and provide a mediated electrochemical oxidation process that efficiently remediates organic and mixed wastes while minimizing the economic and environmental costs of a waste treatment system.